1. Field of the Invention
The present invention relates to heat exchangers, such as those of the type used in automobile air conditioning systems. More particularly, this invention relates to an improved extrusion process for forming ports on a heat exchanger manifold, in which risers are back extruded from the surrounding material of the manifold such that subsequent machining steps to further define and finish the port are unnecessary.
2. Description of the Prior Art
Heat exchangers are employed within the automotive industry as condensers and evaporators for use in air conditioning systems, radiators for cooling engine coolant, and heater cores for internal climate control. In order to efficiently maximize the amount of surface area available for transferring heat between the environment and a fluid flowing through the heat exchanger, the design of the heat exchanger is typically of a tube-and-fin type in which numerous tubes thermally communicate with high surface area fins. The fins enhance the ability of the heat exchanger to transfer heat from the fluid to the environment, or vice versa. For example, heat exchangers used in the automotive industry as air conditioner condensers serve to condense a vaporized refrigerant by transferring heat from the refrigerant to the air forced over the external surfaces of the condenser.
One type of heat exchanger used in the automotive industry is constructed of a number of parallel tubes which are joined to and between a pair of manifolds, creating a parallel flow arrangement. The manifolds form reservoirs that are in fluidic communication with the tubes through tube ports formed in the manifolds. One or both manifolds include one or more inlet and outlet ports through which a coolant enters and exits the heat exchanger. Conventionally, such heat exchangers have been constructed by soldering or brazing the tubes to their respective ports, which may be in the form of risers or openings defined in the walls of the manifolds. Finally, fins are provided in the form of panels having apertures through which the tubes are inserted, or in the form of centers that can be positioned between adjacent pairs of tubes.
The process by which the tube ports are formed has often entailed a significant number of processing steps in order to accurately shape the ports, such that minimal material is employed to achieve a sufficiently strong joint for the intended application. One type of tube port known in the prior art consists primarily of an opening in the manifold wall. While forming such openings generally involves a single punching operation, a drawback of this port configuration is the minimal amount of material available to engage and bond with the tube assembled with the port. This shortcoming is significantly exacerbated if a chamfer is added to the opening to facilitate assembly of a tube. A second type of tube port configuration employed in the prior art overcomes these shortcomings by including a riser or collar that provides a substantially greater amount of material for engagement with the tube. However, risers are more difficult to form than a simple opening in a manifold, and have conventionally entailed multiple forming operations. Accordingly, there is a desire to reduce the steps necessary to form this type of tube port. One such method is disclosed in U.S. Pat. No. 4,663,812 to Clausen, assigned to the assignee of this invention. Clausen teaches forming a longitudinal projection on a manifold, which is then further formed or machined to create solid risers that subsequently undergo a reverse impact extrusion process to form tubular risers. Though the teachings of Clausen provide a greatly simplified method for forming risers, further simplification of the process would be desirable. While processes are known by which a riser can be forged directly from a thick wall of a manifold while the manifold resides within a single die cavity, such methods have necessitated the use of dies whose mating male and female features are prone to excessive wear. One such method is taught by U.S. Pat. No. 5,337,477 to Waggoner, which discloses forging a riser on an oversized manifold by closing a pair of die halves on the manifold. One die half is configured as a punch to cause material flow into a cavity formed by the second die half, such that risers are simultaneously extruded and formed around cores positioned in channels in the second die half. Because the risers are formed entirely by the step of closing the die halves, the die half serving as the punch must project sufficiently into the cavity formed by the mating die half to ensure proper material flow as the dies are closed. The requirement for a closely mating punch and cavity and the resulting high loads that occur during die closure significantly promote wear of the mating die surfaces, and particularly wear of the edges of the punch as it enters the cavity and then engages the manifold. In addition, because the punch causes material throughout the cavity to flow toward the channels in the second die, side loading of the cores tends to occur, producing risers with nonuniform wall thicknesses.
From the above, it can be appreciated that further improvements would be desirable for processes employed to form tube ports on heat exchanger manifolds. In particular, such improvements would preferably minimize the number of processing steps necessary to form a tube port, yet must yield a port that promotes the joint strength of the tube-port assembly.